Telescope Reviews: Oberwerk Ultra 15x70 Tests and Evaluation

I noticed this during my viewing session this morning.

I was focusing on the Trapezium. The Ultras clearly separated the four components of the Trap. I must admit the two closest could easily have been still touching, but because one is slightly brighter, it was easy to see those as two and not one. With a closest components at 8.7", that about the best on-axis resolution I've seen. But this is more about the rest of the field of view.

At about 50%-60% out in the fov are seen the bright stars at the bottom of the sword, and there is a double star at that location. It's a 36" double. At this point out from center, these are already showing some distortion of the point image, but not real bad. The double is still easily seen.

Refocusing on that double (without moving the image in the filed of view), so in this case refocusing on the image at 50%-60% out from center, shows that nearly all of the distortion in the point image can be removed. (Of course, when you do that, the image in the center is thrown out of focus, so the Trap now looks like one blob). That indicates curvature. Curvature is the only aberration that can be focused out. In some instruments curvature is suppressed by a field flattener lens. What we see here in the Ultra is that curvature is present in the lens and it creates about 20 arcseconds of distortion in the point image at a location about 50% to 60% out from center.

I rarely ever refocus my binoculars. Once I'm focused for precise images at the center of field, I leave it alone. Curvature is an aberration that can be focused out, but really, how many would focus it out. Most would just repoint the binocular to center the object of interest and allow whatever is going on in the outer field to just take place.

Out past that point, a few other things start to come into play. Now please keep in mind the variety of aberrations causing image distrotions in the Ultra is not really bad at all. I'm just making some attempt to identify which aberrations are present.

In the outer edges of the field of the Ultra, coma begins to present itself. Stars begin to appear with a brighter point towards the center with a small flared fan shape towards the outer edge. If you could see a ray trace from a single star of all the rays of light formed by the objective, you would see most of the points all hit the same spot. That's the bright point towards the center. But then, some of the rays formed from the outer edges of the objective lens begin to miss the bright spot and get spread out in a fan shape. This results in the classic comet shape. This is fairly typical in binoculars. Seldom do we find a binocular that does not have some coma present. In the Ultra it fairly well controlled, but it is present. It cannot be focused out.

Also in the outer field of the Ultra, we begin to see astigmatism. Astigmatism begins to spread the star point image out in an elongated shape. If coma were not present the star images would look like long lines, generally running parallel to the field stop, sort of like little sections of arcs of a circle. You can tell if it is astigmatism by racking focus in and out past the point of best focus. If the orientation of the "line" flips 90°, it's astigmatism. This also is not uncommon in binoculars. It is present in the Ultra. When astigmatism is combined with coma, the comatic fan gets elongated to the classic seagull, the wings grow longer. Astigmatism cannot be focused out.

I believe I noted in an earlier post that the combined aberrations show the extreme off-axis star image is bloated to about 1 arcminute, not bad at all.

It would be very difficult to tell, but with the presentation of the three aberrations noted above, I did not see any spherical aberration. Maybe just a little. When I focused out the curvature, the image at 50-60% was not as fine as the on-axis precisely focused image, so this could be a little SA. But if so, it is totally overwhelmed by the other aberrations.

So, what's this all mean. Well I've already stated the outer field correction in the Ultra is pretty darned good. This is not a binocular that is suffering from serious aberrations. I'm just taking the time to identify what aberrations are present and about how much they seem to be affecting the image.

Those that want to read into this the fact that aberrations are being reported should watch soon for my observation report from the past several nights. This morning for the first time I can definitively state that I was able to see IC 342. That is a face-on galaxy that, IIRC, has a surface brightness of about Sb mag15.0. Not too shabby at all.

edz